Well logging provides various parameters that may be used to determine the “quality” of a rock formation from a given wellbore. Hydraulic permeability, a measure of how easily the fluids will flow through the pores of the formation, is a particularly important factor in determining the commercial viability of a specific well site. One of the most direct methods of measuring permeability is by measuring fluid flow rates in the formation. Because flow rates are difficult to determine, NMR measurements of flow rate can be used to assist in the process of modeling the flow rates and thereby determine the permeability of the formation. Accordingly, NMR is emerging as an invaluable tool for the characterization of formations in geophysical exploration, both for well logging and logging-while-drilling (LWD).
There are two types of flow that occur in boreholes—either natural flow between the formation and the borehole, or induced flow resulting from pumping fluids out of the formation by a Modular Formation Dynamics Tester (MDT)™ or similar tool. Most conventional applications of downhole NMR flow analysis are limited to situations in which the flow is induced by external means, such as the MDT tool. There are a number of circumstances, however, in which flow occurs naturally due to a pressure differential between the formation and the wellbore.
Natural flow typically occurs in three situations: Under-Balanced Drilling (UBD) conditions, wells with open-hole completions, or cross-flow between formation zones along the trajectory of the wellbore. In the case of UBD, the wellbore pressure is kept below the reservoir pressure, which allows the reservoir fluids to enter the wellbore during drilling and eliminates invasion of the drilling fluids into the formation. UBD is becoming increasingly popular because of the many advantages over the conventional over-balanced drilling.
Open-hole completion is a process of well completion that has no casing or liner set across the reservoir formation, allowing the produced fluids to flow directly into the wellbore. Natural flow occurs during reservoir production when the wellbore pressure is reduced below formation pressure and the fluids flow naturally from the formation into the borehole. Many horizontal wells created in competent rock are completed open-hole.
The third case of natural flow occurs when two layers along the wellbore are at significantly different pressures. In this situation, there is no significant flow out of the bore hole, yet there is cross-flow between the different layers. It is quite common in water flood operations to have significant pressure differentials between different zones (e.g., greater than 3000 p.s.i.). Although the well is actually flowing downhole (or more precisely, cross flowing), this flow is usually not recognized during drilling because all of the fluid moves from the high pressure flooded zone, into the wellbore, and then out of the wellbore into a low pressure zone (e.g., where the water flood is not present).
In UBD conditions, wells with open hole completions, wells exhibiting cross flow between layers of higher versus lower formation pressure, and other natural flow conditions, fluid flow measurements would be made using the natural flow of the fluids between the formation and the borehole. These flow measurements would provide for the estimation of a number of continuous flow based properties, including, but not limited to, matrix permeability measurements along the borehole and relative permeability in a continuous manner along the borehole. In natural flow conditions, NMR flow measurements, when combined with production logging methods, would also provide the positive identification and contribution of water or hydrocarbon inflow from natural fractures. Accordingly, there is a need for monitoring natural fluid flow resulting from a pressure differential between the formation and the borehole.
Conventional NMR scans can be considered as taking place in two steps. The first step is the manipulation of nuclear spins, for example, by a series of RF and magnetic field gradient pulses. In the second step, the resulting spin magnetization is detected. The first step is often called the encoding step since it is meant to modulate the spins in certain pattern. Usually, a systematic variation of the encoding segment of the sequence is executed and corresponding signal measured. Analysis of the signal as a result of the known pattern of modulation allows the extraction of the properties of the nuclear spins and the spin-containing materials.
Although NMR has developed into a very versatile analytical tool, NMR is a relatively insensitive detection method compared to others because the NMR signal depends on the population difference between two spin states. Unfortunately, many approaches to improving NMR sensitivity result in optimizing the encoding process at the expense of the detection process, or vice versa.
Therefore, it is a desire to provide a system and method of using NMR analysis to measure natural fluid flow under well logging and LWD conditions. Furthermore, there is a need for a system and method for NMR that can optimize both the encoding and detecting procedures and overcome the inherent limitations of traditional NMR devices.